Same process worked for me, pretty nastly strain free from petstore, serves me right for puttin them straight in, was my first fishy buy, killed 1 loach, well stressed him and something else got him.
Used higher temp and good water flow, only put to 30 degrees for 2 days and 28 deg for 15 after that. Never come back since, 6 months and counting. Fishies could not take 30 for long but 2 days and 28 for 17 after that was enough for the problem I had. That and lots of gravel cleaning water changing, can also get a 1 micron sponge for your filter, free floaters are 3-4 microns or more I think?
Stagnation at any time in your tank, manky
Mark